Low density porous materials useful in sound and/or thermal insulation applications, shock absorbing applications, and the like, have generally been low cost petroleum-based materials such as foamed styrols (polystyrenes). Unfortunately, these materials are primarily developed from non-renewable resources. In addition, they present multiple disposal problems. For example, polystyrenes are incinerated at high temperatures that may damage containment vessels and generate a large amount of CO2. Furthermore, if such polystyrenes are not incinerated at extremely high temperatures, the process may generate polycyclic aromatic hydrocarbons (PAH). If such polystyrenes are not incinerated, they may remain intact as pollutants for an extremely long period of time.
In an attempt to provide alternatives to petroleum-based materials, composites have been developed that utilize more eco-friendly raw materials. For instance, inorganic materials such as silicas, clays, metal oxides and the like have been formed into aerogels. Clays have attracted a great deal of interest due to their abundance, environmental safety, and physical characteristics. Many such composite systems utilize layered or smectic clays that may be exfoliated into individual layers. For example, U.S. Patent Application Publication No. 2007/0208124 to Schiraldi, et al. describes a clay aerogel/polymer composite including exfoliated clay and polymers such as starches, plant gums, modified cellulosic and lignin materials, and the like.
Exfoliation of layered clays may be used to form nanoclays. Nanoclays are a broad class of inorganic minerals, of which plate-like montmorillonite is the most commonly used in materials applications. Montmorillonite consists of roughly 1 nanometer (nm) thick aluminosilicate layers surface-substituted with metal cations and stacked in about 10 micrometer (μm) thick multilayer stacks. The exfoliated layers may be dispersed in a polymer matrix to form a polymer-clay nanocomposite. Within the nanocomposite, individual clay layers form plate-like nanoparticles with a very high aspect ratio. Even at low nanoclay loading, the majority of the polymer chains may be held in close contact with a clay surface. This may dramatically alter properties of the nanocomposite as compared to an unfilled polymer matrix. For example, a nanoclay/polymer composite may exhibit increased mechanical strength, decreased gas permeability, superior flame-resistance, and even enhanced transparency as compared to an unfilled polymer matrix. For instance, U.S. Pat. No. 7,553,898 to Rafailovich, et al. describes a flame retardant plastic mixture that includes a polyolefin, a brominated polystyrene or decabromodiphenyl ether, a nanoclay, and metal oxide fillers.
Unfortunately, the potential of nanoclay composite materials has yet to be fully attained. This is understood to be primarily due to the tendency of clay nanoparticles to self-assemble into the relatively large, well-ordered lamellar structures found in nature. Nanocomposites formed to date tend to exhibit a lamellar structure including a large proportion of macroscopic porosity caused by the formation of these lamellar structures, which may prevent realization of desired composite characteristics.
What are needed in the art are composite materials and methods for forming the composites that provide favorable thermodynamic driving forces so as to overcome the tendency of nanoclay particles to assemble into lamellar structures. Such methods may be utilized to form eco-friendly, low density, fire-resistant composite materials that exhibit a homogeneous microscopic porous structure and desirable physical characteristics.